1. Field of the Invention
This invention relates to integrated circuits (ICs) for controlling the functions of a cordless telephone and, more particularly, to an IC with a dual-mode baseband controller for radio-frequency (RF) interfaces.
2. History Of the Technology
The Radio Equipment and Systems (RES) Technical Committee of the European Telecommunications Standards Institute (ETSI) has developed an Interim European Telecommunication Standard (I-ETS). This standard, I-ETS 300 131R1, dated 3 Feb. 1993, Fifth Draft, is titled "Radio Equipment and Systems (RES); Common air interface specification to be used for the interworking between cordless telephone apparatus in the frequency band 864.1 MHz to 868.1 MHz, including public access services," and is hereby incorporated by reference herein.
The I-ETS 300 131 R1 specification covers the minimum performance requirements for fixed and portable radio units used with the second generation cordless telephone (CT2), common air interface service (CAI), operating in the band 864.100 MHz to 868.100 MHz.
CT2 cordless telephones use a time division duplex 32-kbit/sec Adaptive Differential Pulse Code Modulation (ADPCM) voice (B) channel and a 1-, 2-, or 16-kbit/sec control (D) channel between the handset and the base station. The physical implementation is a 72-kbit/sec ping-pong type radio link with identical transmit and receive frequencies. In countries where the frequency band is available, the channel center frequencies for the forty (40) CT2 channels are 864.050 MHz+(0.100.times.n) MHz, where n is the channel number lying in the range 1 to 40, inclusive. The first channel (channel no. 1) lies at 864.150 MHz and the last channel (channel no. 40) lies at 868.050 MHz.
The channel frequency accuracy required of both the base and handset of the cordless telephone is .+-.10 kHz, maximum difference between the nominal and actual channel center frequencies over specified supply voltage and temperature ranges. Automatic frequency control (AFC) may be used in the receiver at both the base and the handset, but may only be linked to control the transmitter center frequency at the handset. The maximum rate of change of the transmit center frequency at both the base and the handset cannot exceed 1 kHz/ms, except for the specific cases of switching of the handset transmitter from Signaling Multiplex Mode 3 (MUX3) to Signaling Multiplex Mode 2 (MUX2) and for channel changing.
There are two popular radio frequency transmitter architectures utilized in CT2 digital cordless telephones to meet these transmitter signal specifications. The first is In-Phase/Quadrature (I-Q), and the second is Non-Return to Zero (NRZ). Each of these radio frequency transmitter architectures has distinct advantages and disadvantages as discussed below.
The I-Q architecture generally allows tighter spectral control and reduces the number of tuning devices needed in the radio transmitter. In the manufacturing process, a greater number of tuning devices generally equates to higher labor costs. Therefore, utilization of the I-Q architecture, which reduces the number of tuning devices, reduces labor costs relative to the NRZ architecture. Disadvantages of the I-Q architecture include relatively high costs of radio components utilized in the I-Q architecture, and relatively high power consumption characteristics.
The NRZ architecture utilizes standard super-heterodyning techniques, taking advantage of components which have been cost-reduced due to enormous commercial radio industry production volumes. Heterodyne reception is the process of reception in which a received high frequency wave is combined in a non-linear device with a locally generated wave. The process normally occurs in a frequency converter in which the signal input frequency is changed by superimposing a local oscillation to produce an output having the same modulation information as the original signal, but at a frequency which is either the sum or the difference of the signal and local oscillator frequencies. In super-heterodyne reception, the process of heterodyne reception is used to convert the voltage of the received wave into a voltage of an intermediate, but usually super-audible frequency, that is then detected.
The disadvantages of the NRZ architecture are that the spectral characteristics are relatively poor when compared to the I-Q architecture, and the NRZ architecture requires relatively more tuning devices, thereby increasing production labor costs.
In the past, integrated circuits (ICs) for controlling the functions of cordless telephones have included the capability to support either the I-Q architecture or the NRZ architecture, but not both. The controlling ICs have TX+ and TX- pins, and the two RF transmitter architectures require different waveforms at the TX+ and TX- pins, as well as different timing at various control pins of the IC. It would be a distinct advantage to have an IC capable of supporting either the I-Q architecture or the NRZ architecture, whichever is chosen by the ultimate customer. This would allow overall production costs to be reduced through volume production of one design rather than two. It also offers the ultimate customer an easy migration path from the lower performance NRZ architecture to the higher performance I-Q architecture. The present invention provides a radio frequency (RF) interface circuit which enables a single IC to support either the I-Q or the NRZ radio frequency transmitter architecture.